Semiconductors such as AlN, GaAs, GaN, InP, Si and SiC can be formed by deposition. Examples of deposition techniques include chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). In such a deposition technique, a film can be formed such that a substrate is placed in an evacuated chamber and source molecules are supplied in the form of a source gas onto the substrate to deposit a crystal layer on the surface of the substrate.
In deposition techniques of this type, the temperature of the substrate in the chamber has to be accurately controlled in order to form a high purity, high density, reproducible semiconductor crystal layer at a constant deposition rate. To this end, a monitor for measuring the temperature of the substrate in the chamber is provided along with a heater for heating the substrate, so that the heating temperature of the heater can be controlled based on the temperature measured by the monitor.
Conventionally, as described in Patent Literatures 1 and 2 cited below, a pyrometer for monitoring an infrared light that will be emitted from the surface of the heated substrate has been used as the monitor. The pyrometer is disposed outside a window of the chamber so that the infrared light emitted from the surface of the substrate or the surface of the semiconductor layer during deposition can be detected by the pyrometer through the glass window. However, the temperature monitoring by the pyrometer has the following problems.
When the infrared light emitted from the surface of the heated substrate passes through the semiconductor layer during deposition, a light passing through the semiconductor layer interferes with a light reflected inside the semiconductor layer to cause minor fluctuations of the detection output from the pyrometer, and moreover, the degree of interference varies with a change in the film thickness of the semiconductor layer during deposition. Conventionally, this problem has been solved by disposing a light emitting device outside the chamber, applying a laser light to the semiconductor layer during deposition through the glass window of the chamber, and monitoring a laser light passing through the semiconductor layer. Since the laser light passing through the semiconductor layer also interferes with a laser light reflected inside the semiconductor layer, as with the case of the infrared lights, the output fluctuations of the monitored laser light due to the interference can be used for calibration to eliminate or reduce the interference with the infrared light to be detected by the pyrometer.
However, even if the interference with the infrared light to be detected by the pyrometer can be avoided, the temperature monitoring is performed by the pyrometer at a place away from the surface of the substrate, generally, outside of the chamber through a glass window. Since not only a long distance but also the glass window exists between the substrate surface from which heat is actually emitted and the monitoring spot, it is inevitable that an error will arise between the temperature measured by the pyrometer and the actual temperature of the substrate surface.
If the semiconductor layer growing on the surface of the substrate is transparent, moreover, the pyrometer actually measures the temperature of the substrate surface through the transparent semiconductor layer. Thus, it is difficult to directly and accurately measure the temperature of the growing semiconductor layer itself by a measuring method with a pyrometer.
Patent Literature 1 cited below further discloses the use of a thermocouple monitor for measuring the temperature of the substrate at its back side. However, since the thermocouple monitor is disposed on the back side of the substrate, it is impossible to accurately measure the actual temperature of the substrate surface. In addition, since the thermocouple monitor cannot readily respond to a temperature change in the chamber because of its large heat capacity, it is difficult to accurately measure the temperature of the substrate.
Patent Literature 3 cited below further discloses a technique of irradiating a light from a halogen lamp on a wafer to be measured and calculating the surface temperature of the wafer from transmittance, reflectance and wavelength of the light.
However, since transmittance and reflectance of light vary greatly depending on various factors such as surface roughness of the wafer, it is difficult to determine the temperature of the object to be measured with high accuracy only from transmittance and reflectance of a single light.